Multiple beam cathode ray tubes are frequently used to display alphanumeric and/or other types of visual pattern information. Each of the multiple beams concurrently produces scan lines on the face of the tube and consequently, such tubes have a greater bandwidth than single beam tubes, which enables them to display more information at suitable brightness than a single beam type of tube.
Typical multiple beam cathode ray array tubes utilized in the prior art arrange a plurality of closely spaced cathodes in a vertical column array (collinear) to produce a vertical column array of closely spaced electron beams. Accelerating means, focusing means and deflection means are disposed within the envelope of the cathode ray tube or surrounding same. Normally, the individual beams are accelerated, focused and deflected across the screen and are repeatedly being turned on and off with a suitable video signal to form dots on the screen at appropriate scanning locations. It is well known to form the desired character or other pattern, utilizing logic circuitry within the video portion of the system to selectively control each beam to be either on or off at various scanning positions, and the resulting arrangement of variable intensity dots forms the desired pattern. A general problem encountered with multiple beam cathode ray tubes is that of off-axis aberrations or distortions. Since only one beam can be emitted along the axis of the tube, the remainder of the beams in the multiple beam tube are off-axis by varying amounts. The distortions or aberrations are caused by nonuniformities in the deflection and focusing fields, and these nonuniformities cause the distortions in the projected beams to increase with distance from the axis.
According to known electronic principles, in conventional multiple beam tubes, beams are emitted parallel to the axis and are accelerated in the same direction to the focusing means or lens, which changes the direction of the beams and causes them to converge toward a cross-over point which is normally located in the funnel portion of the tube.
In prior art collinear multiple beam cathode ray tubes, parallel beams are spaced from each other by a substantial distance, resulting in a relatively large maximum off-axis distance as the beams traverse the focusing means, and due to the fact that the beams do not cross until they are well into the funnel portion of the tube, a relatively large amount of off-axis distance results as the converging beams traverse the deflection means. The magnetic deflection yoke is the component in such systems which introduces the largest single aberration due to fringing fields and the like, and this distortion is most severe when a large deflection angle is utilized in the tube which permits the length of the tube to be minimized for a given screen size. The off-axis aberrations caused by such conventional arrangements as described above make it very difficult to focus the beams at all locations on the screen and have proven to be quite troublesome.
In addition to problems of focus, such multiple beam cathode ray tubes suffer from two other well known distortions. These are shear and rotation. Shear is in effect a quadrature distortion and results in a distortion of the projected matrix wherein a compression is caused along one axis of the matrix accompanied by an expansion along the other. Thus, a graphical illustration of shear distortion is to consider a square matrix of beams being projected upon the screen. Due to the shear, the projected matrix would not be square. Thus, the shear distorted square would be forced into a rhombus, and in another form of shear distortion the square might be converted into a nonequilateral parallelogram or rectangle. Quadrature compensation stigmators or quadrapoles have been used in prior art systems. In prior art collinear multiple beam cathode ray tubes, shear distortion is indistinguishable from rotation of the linear array on the screen of the tube. The quadrapole correction currents could usually be adjusted to achieve reasonable correction of this form of distortion.
With multiple beam cathode ray tubes which actually project a two-dimensional matrix type of array on the screen, quadraple shear correction does not correct for actual rotation of the complete matrix caused by traversing the focusing and deflection coil.
Two-dimensional matrix array beams are known in the art to be more desirable than a linear array due to the fact that the individual cathode and other beam forming structures can be spaced a greater distance apart within the cathode or electron beam emissive structure to allow for the formation of a much narrower and better defined beam without interference from other nearby structures. Further, because the beams are very close together in a collinear array, and may actually touch each other, mutual beam repulsion results, which may cause the top and bottom beams to be deflected upwardly and downwardly, respectively when the beams are turned on. Also, since the beams are located very close to each other, there is little space to build and mount the grids which control the intensity of the beams. Finally, the closeness of the beams places an effective limit on the amount of current which each beam may contain and also results in beam intermodulation, wherein the control grid of one beam may affect or intermodulate the current of another beam, thereby precluding effective grid control. The above problems are obviated by a matrix electron beam array instead of a collinear array.
Thus, a 4.times.4 matrix array may be utilized to form sixteen very closely spaced scan lines by rotating the matrix a predetermined amount so that the horizontal scan lines produced by the beams are equally spaced. Suitable delays may be introduced in the individual beam modulation circuits to, in effect, present a vertical scan line across said screen. To the observer, there appears to be a vertical line scan by all sixteen beams. Such matrix arrays can undergo rotation and shear distortions which are distinguishable. In the single beam or collinear case shear and rotation are indistinguishable.